Memory Protection project interface details (GSoC 2020)

Tue May 12 03:57:08 UTC 2020

On Thu, May 7, 2020 at 9:59 PM Hesham Almatary
<hesham.almatary at cl.cam.ac.uk> wrote:
>
> Hello Utkarsh,
>
> I'd suggest you don't spend too much efforts on setting up BBB
> hardware if you haven't already. Debugging on QEMU with GDB is way
> easier, and you can consider either qemu-xilinx-zynq-a9 or rpi2 BSPs.
> Later, you can move your code to BBB if you want, since both are based
> on ARMv7.
+1

Past work has also used psim successfully I thought? Or am I mistaken there.

>
> On Thu, 7 May 2020 at 18:26, Utkarsh Rai <utkarsh.rai60 at gmail.com> wrote:
> >
> > Hello,
> > This is to ensure that all the interested parties are on the same page before I start my project and can give their invaluable feedback.
Excellent, thank you for getting the initiative.

I'll be taking on the primary mentorship for your project, with
support from the co-mentors (Peter, Hesham, Sebastian). For now, I
prefer you to continue to make your presence on the mailing list. We
will establish other forms of communication as needed and will take on
IRC meetings once coding begins in earnest.

> > My GSoC project, providing user-configurable thread stack protection, requires adding architecture-specific low-level support as well as high-level API support. I will be starting my project with ARMv7-A (on BBB) based MMU since RTEMS already has quite mature support for it. As already mentioned in my proposal I will be focusing more on the High-level interface and let it drive whatever further low-level support is needed.
> > Once the application uses MMU for thread stack address generation each thread will be automatically protected as the page tables other than that of the executing thread would be made dormant. When the user has to share thread stacks they will have to obtain the stack attributes of the threads to be shared by pthread_attr_getstack() and then get a file descriptor of the memory to be mapped by a call to shm_open() and finally map this to the stack of the other thread through
> > mmap(), this is the POSIX compliant way I could think of. Now at the low level, it means mapping the page table of the thread to be shared into the address space of the executing thread. This is an area where the low-level support has to be provided. At the high-level, this means providing support to mmap and shared-memory interface as mmap provides support for a file by simply
> > copying the memory from the file to the destination. For shared memory objects it can
> > provide read/write access but cannot provide restriction of write/read access. One of the areas that I have to look into more detail is thread context-switch, as after every context switch the TLBs need to be flushed and reinitialized lest we get an invalid address for the executing thread. Since context-switch is low-level architecture-specific, this also has to be provided with more support.

This is really dense text. Try to break apart your writing a little
bit to help clarify your thoughts.  You should also translate some of
your proposal into a wiki page if you haven't started that yet, and a
blog post. Both of those will help to focus your thoughts into words.

"mapping the page table" is not meaningful to me. I think you mean
something like "mapping a page from the page table"?  Will the design
support sharing task stacks using MPUs with 4 regions? 8?  (It seems
challenging to me, but might be possible in some limited
configurations. Having support for those kinds of targets might still
be useful, with the caveat that sharing stacks is not possible.)

The first step is to get a BSP running that has sufficient
capabilities for you to test out memory protection with. Do a little
bit of digging, but definitely simulation is the way to go.

The second step from my perspective is to determine how to introduce
strict isolation between task stacks. Don't worry about sharing at
this stage, but rather can you completely isolate tasks? Then you can
start to poke holes in the isolation.

As you say, you'll also need to start to understand the context switch
code. Start looking into it to determine where you might think to
implement changing the address space of the executing thread. Another
challenge is that RTEMS can dispatch to a new task from the interrupt
handler, which may cause some problems for you as well to handle.

Have you figured out where in the code thread stacks are allocated?
How do you plan to make the thread stacks known to other threads?

TLB shootdown can be extremely expensive. Try to find ways to optimize
that cost earlier rather than later. (One of those cases where
premature optimization will be acceptable.) Tagged TLB architectures
or those with "superpages" may incur less overhead if you can
selectively shoot-down the entry (entries) used for task stacks.

A final thought is that the method to configure this support is
necessary. Configuration is undergoing some heavy changes lately, and
application-level configuration is going to be completely different in
rtems6. You may want to consider raising a new thread with CC to
Sebastian to get his input on how the best way to configure something
like this might look, now and in the future. I would have leaned
toward a high-level configure switch (--enable-task-protection) in the
past, but now I don't know.  This capability is however something that
should be considered disabled by default due to the extra overhead.

Gedare

> > Kindly provide your feedback if I have missed something or I have a wrong idea about it.
> >
> > Regards,
> > Utkarsh Rai.
> >
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